An available taxi is useful to passengers even while it is carrying nobody. More idle vehicles spread through a service area make pickup faster, reducing the delay between requesting a ride and entering a car. Keeping those vehicles available, however, costs driver time and money.
Too few ready cars leave riders waiting, but expanding the available fleet also raises operating cost. As idle coverage grows, the distance to a nearby car falls and passengers spend less time waiting for pickup. The gain becomes less dramatic as the fleet gets larger, while the cost of idle capacity continues. This is the model's central tension: empty vehicles are not pure waste, because they provide an option to serve future riders quickly. Efficient ride service requires balancing that option value against the resources tied up in vehicles waiting for work.
One ride has two different clocks. For the passenger, the trip begins with a request and includes waiting to be matched, waiting for the assigned car to arrive, and riding to the destination. For the driver, the service cycle begins while available, continues through pickup and transport, and ends only when the car is ready for another match.
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An on-demand car, a shared ride, and a fixed-route bus use vehicles very differently. An exclusive car offers direct travel but spends substantial time and distance on each passenger. Pooling shares some of that work at the cost of detours and coordination. A bus gains scale when enough people use a common corridor, but offers less flexible access.
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Private autonomous vehicles may sit unused for much of the day while other people cannot find a ride. A platform could recruit some of those vehicles, pay their owners for availability or service, and charge passengers for completed trips. Supply would then come partly from assets already owned by households.
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